1. Field of the Invention
The present invention relates to a color processing apparatus and method and, more particularly, to a color processing apparatus and method, which convert input spectral information into an output signal value of an image output device.
2. Description of the Related Art
In recent years, in the field of color reproduction techniques that attain color reproduction of target objects, a spectral based color reproduction technique has been studied and developed. The spectral based color reproduction technique reproduces spectral information itself as physical information of the colors of a target object unlike a conventional colorimetric color reproduction technique that matches XYZ values, L*a*b* values, or the like to the color of a target object to adjust color based on metamerism. That is, when spectral based color reproduction is applied, the color of a target object can be faithfully reproduced independently of illumination environments upon observing the target object, personal differences of human visual characteristics, and the like, as if the target object were in the face of a user. Therefore, this color reproduction technique is expected to be widely applied to the fields that require faithful color reproduction of target objects, for example, in the fields such as the medical field, digital archiving of arts, on-line shopping and industrial design using the Internet, and the like.
As a technique for acquiring spectral information of a target object, a multi-band camera using color filters of four or more bands has been developed. By capturing an image of the target object using this multi-band camera, a spectral image each pixel of which has spectral information can be acquired.
On the other hand, as a technique for outputting the spectral image, a technique for calculating an output signal value of an image output device by applying optimization to spectral information of each pixel of an input image is known (for example, see PCT(WO) No. 2005-508125 (patent reference 1)). Also, a method of creating a lookup table (LUT) by analyzing spectral information of an input image, and converting the spectral information into an output signal value of an image output device while preserving the spectral information is available (for example, see Japanese Patent Laid-Open No. 2002-254708 (patent reference 2)). In addition, a method of attaining color reproduction using an LabPQR space defined by adding spectral information to tristimulus value information in the conventional colorimetric color reproduction is proposed.
However, when a spectral image is converted into output signal values of an image output device and the converted output signal values are to be output, a processing time required to convert the spectral image into the output signal values poses a problem in addition to reproduction precision of the input spectral information.
For example, when the output pixel value of the image output device is calculated by applying optimization processing to each pixel like the technique described in patent reference 1, the reproduction precision of spectral information can be improved. However, since the optimization processing is applied to all pixels, a very long processing time is required.
According to the technique described in patent reference 2, by creating an LUT for converting spectral information of an input image into output signal values of an output device while temporarily holding the spectral information, high-speed conversion into output signal values of the output device can be attained. However, when a different image is input, an LUT needs to be created again, thus consequently requiring a processing time to create an LUT.
Furthermore, when the LabPQR space defined by adding spectral information to tristimulus value information is used like in the technique described in non-patent reference 1, relatively high-speed conversion processing can be accomplished. However, the reproduction precision of spectral information is impaired.
As described above, according to the conventional spectral based color reproduction techniques, the color reproduction precision and processing time have a tradeoff relationship, and it is difficult to improve both of them.